In recent years, a rapidly increasing volume of traffic due to the spread of the Internet has been pressing a communication band of trunk system communication systems.
When a communication band of a communication system is pressed, congestion occurs frequently to cause a problem that communication is allowed intermittently or a problem that communication stops completely or the like.
Demanded accordingly is a technique enabling continuous provision of stable communication by expanding a transmission capacity of a communication system.
In addition, because when in such a trunk system communication system as transferring a large volume of traffic, occurrence of such abnormality as link cut-off or a network node (hereinafter simply referred to as a node) failure exerts a large effect in a wide range, demanded is a highly reliable technique of a communication system which enables stable communication to be continued even when such abnormal situation as described above occurs.
Generally widely known as a network protocol which realizes such a stable communication system as described above is a Resilient Packet Ring (RPR) disclosed in IEEE Standards 802.17 (Literature 1) as standardization document issued by IEEE (Institute of Electrical and Electronics Engineers) in 2004.
RPR is a network protocol for transferring a frame on a network having such ring topology as shown in FIG. 25.
A communication system shown in FIG. 25 is an example in which with an RPR network (hereinafter referred to as an RPR network) formed by four nodes operable based on RPR (hereinafter referred to as an RPR node), one terminal node is accommodated under the control of each RPR node.
Known as a main characteristic of RPR is a high-speed protection function.
In a case, for example, where in an RPR network, when a link between RPR nodes is cut off, immediately after RPR nodes on the opposite sides of the link detect the cut-off, all the RPR nodes are notified to that effect instantly.
The RPR node being notified is allowed to continue communication because traffic is transmitted so as to detour the link cut-off part.
Since RPR is designed to recover communication within as short a time period as 50 ms equal to that of SDH (Synchronous Digital Hierarchy) or SONET (Synchronous Optical Network) premised on adoption to a trunk system communication system in which a large volume of traffic flows as in a city network, it is possible to set up a highly reliable communication system.
When the volume of traffic exceeds a transmission capacity originally presumed at the set-up of an RPR network to press a communication band of the RPR network, a simplest and common method as a method of increasing a transmission capacity of the already set-up RPR network is newly setting up an RPR network having a larger transmission capacity by using an RPR node with a communication interface using an optical part and an electric part operable at a higher bit rate mounted and then shifting to the new RPR network.
Also possible is a method of applying the technique disclosed in Japanese Patent Laying-Open No. 10-285200 (Literature 2) to an RPR network.
More specifically, with a plurality of RPR networks connected by a relay node and with a network formed to have a terminal disposed under the control of a relay node as well, the relay node distributes and transfers a data frame received from the terminal to any of the plurality of RPR networks.
With the foregoing arrangement, traffic received from the terminal is distributed and transferred to the plurality of RPR networks to enable a transmission capacity of the RPR network as a whole to be increased.    Literature 1: “RPR (Resilient Packet Ring)”, IEEE Standards 802.17, IEEE, 2004.    Literature 2: Japanese Patent Laying-Open No. 10-285200.    Literature 3: Japanese Patent Laying-Open No. 2004-140776.
When increasing a transmission capacity of the RPR network, however, the above-described related art has the following problems.
In a case of the method of shifting to a communication system having a larger transmission capacity which is formed of a node with a communication interface using an optical part and an electric part operable at a higher bit rate mounted, the node mounted with the high-speed interface is extremely expensive to require enormous costs for setting up a new RPR network.
When a future change in the volume of traffic is hard to estimate, in particular, since there is no other choice than to use a node having a communication interface operable at the maximum speed among usable communication interfaces, communication system set-up will cost more.
Even when a future change in the volume of traffic can be estimated, because speed of the communication interface lacks diversity, a communication system whose transmission capacity is far larger than a desired transmission capacity should be set up, so that extra cost will be required.
In addition, in terms of reliability of a communication system, in a case of RPR, when an RPR node forming an RPR network develops a fault, while communication between terminals under other RPR node than the RPR node in question can be maintained, communication between a terminal under the failing RPR node and other terminal can not be maintained.
On the other hand, as disclosed in Literature 2, when the transmission capacity of the RPR network is increased by the method of distributing traffic to a plurality of RPR networks by a relay node, between the relay node and the RPR network, and between the relay node and the terminal are each connected by only one link and besides, the relay node itself is not structured to be redundant.
In other words, as architecture for setting up a trunk communication system, it can not help saying that it lacks reliability.
Literature 2 fails to disclose a data frame transfer method executed when in any of a plurality of RPR networks connected to a relay node, such abnormality of cut-off of communication between at least two RPR nodes among the RPR nodes forming the RPR network occurs as a failure of an RPR node.
An object of the present invention, in view of such problems of the related art as described above, is to provide a highly reliable communication system whose desired transmission capacity can be realized at low costs and which can be restored from an abnormal state in a short time period.